Speech transmission system



May 12, 1942. E. M. STAPLES SPEECH TRANSMISSIQN SYSTEM Filed Feb. 25, 1941 0 k 9 v M9 3 Q is TI- 2 Sheets-Sheet l PHANTOM l SIDE CCZ' lNVENTO/P M. S771PLES May 12, 1942.

EASF- l I I l I I I I I I I WEST 3/05 ccr E. M. STAPLES 2,282,434

SPEECH TRANSMISSION SYSTEM Filed Feb. 25, 1941 '2 Sheets-Sheet 2 PI'MN TOM lNVEA/TOR Y EM STAPLES ATTORNEY iatented May 12, 1942 SPEECH TRANSMISSION SYSTEM I Elliot M. Staples, Hohokus, N. J., assignor to Bell Telephone Laboratories,

Incorporated,

' New York, .N. Y.,. a corporation of New York Application February 25, 1941, Serial No. 380,426

- 4 Claims. (01. 179-3) This invention relates to a speech transmission system of the type which includes telephone repeaters spaced at intervals along the transmission line and more particularly to such a system adapted to transmit, in addition to the speech currents, direct current signaling currents.

An object of the invention is to increase the efiiciency of speech transmission systems.

A more specific object of the invention is to prevent pulse distortion due to so-called crossfire resulting from application of direct current signaling currents to the speech transmission lines. (Cross-fire is a term commonly used by communication engineers as referring to disturbing currents that are set up due to mutual admittance in one circuit when direct current signals are passed through a paralleling circuit.)

A copending application in the names of P. G. Edwards and E. R. Taylor, Serial No. 380,446, filed Feb. 25, 1941, entitled Speech transmission system, discloses, among other things, a novel way of applying direct current signaling currents to a speech transmission system. Briefly stated here, but described in detail in the copending application referred to, the telegraph (direct current) signaling currents are applied to the speech transmission lines in shunt with capacitances located between the line windings of the line repeating coil whereby the direct current composite sets are shunted by a low impedance at voice frequencies. The arrangement contemplated by the present invention is particularly applicable to the above arrangement as fully disclosed by the copending application referred to.

A feature of the present invention comprises means effective to materially reduce the coupling at low frequencies beiwveen the two sections of the secondary winding of the line repeating coil without materially reducing the efiiciency of the coil at speech frequencies.

In accordance with a specific embodiment of the invention the drop side of the line repeating coil is shunted by an inductance. This in effect reduces the efficiency of the repeating coil at low frequencies and materially reduces low frequency cross-fire resulting from passage of direct current telegraph currents through either section of the line repeating coil secondary. The provision of the inductance results in no material reduction in the efficiency of the coil at voice frequencies, however.

A complete understanding of the novel arrangements contemplated by the present inveneast amplifier 25.

tion as well as appreciation of the Various desirable features thereof will be gained by consideratlon of the following detailed description in connection with the annexed drawings in which:

Fig. 1 illustrates schematically a speech transmission system which embodies features of the present invention; and

Figure 2 illustratesschematically a modified form of terminal for use with the system of Fig. 1.

Referring now to Fig. 1, there is illustrated schematically a speech transmission system of the general type disclosed in the copending application of P. G. Edwards andE. R. Taylor referred to above. As illustrated, the system comprises a west terminal, an intermediate repeater section and an east terminal. It will be understood, however, that the system" may, and in most cases will, include more than. one. intermediate repeater section between the terminals.

Speech currents produced by the voice frequency apparatus associated with the West terminal of side circuit No. 1, which apparatus would actually be a telephone set but is here represented by its nominal resistance, taken as 600 ohms, pass over lines l2 and I3 to repeating coil l4 and pass therethrough to lines l5 and 1'6 for transmission to the intermediate repeater section.

Upon reaching the intermediate repeater section the voice frequency currents pass through repeating coil H to line section 2|. The voice frequency currents divide in line section 2|, one half passing by induction into output path 22 of east-West amplifier 23 and being lost and the useful half passing into input path 24 of west- After amplification by westeast amplifier 25, the currents pass by induction into line section 26 Where they again divide, one half being absorbed by balancing network 2'! and the useful half passing through repeating coil 4] to lines 42 and 43 for transmission to the east terminal.

Upon reaching the east terminal, the voice frequency currents pass through repeating coil 44 to lines 45 and 46 over which they pass to the receiving equipment of the Voice frequency apparatus associated with the east terminal of side circuit No. 1, which apparatus is represented by resistance 41 which may be of the order of 600 ohms.

It will be understood that transmission of voice frequency currents over side circuit No. 1 from east to west and over side circuit No. 2 in both directions is accomplished in a manner similar to that described above.

Voice frequency currents produced by the voice frequency apparatus associated with the west phantom terminal, which apparatus is represented by resistance I which may be of the order of 600 ohms, pass through repeating coil 52 to a phantom circuit derived over lines 53 and 54. Line 53 terminates in side circuit No. 1 between capacitances 55 and 56 which are connected, in turn, between line windings 51 and II of repeat ing coil I4 while line 54 terminates in side circuit in No. 2 between capacitances I2 and I3 which are connected, in turn, between line windings I4 and of repeating coil I6.

At the intermediate repeater section, the phantom voice frequency currents are picked oil. over lines TI and BI and applied through repeating coil 82 to line section 83. After amplification by phantom west-east amplifier 84 (in a manner similar to that described above) the phantom speech currents are reapplied to the phantom circuit, over lines 85 and 85, for transmission to the east terminal.

Upon reaching the east terminal the phantom voice frequency currents are picked off over lines 37 and I8! and applied through repeating coil I52 to lines I03 and I04 for transmission to the receiving equipment of the east terminal phantom voice frequency apparatus, which apparatus is represented by resistance I05 which may be of the order of 600 ohms.

It will be understood that transmission of voice frequency currents over the phantom circuit from east to west is accomplished in a manner similar to that described above.

Direct current signals, for example telegraph signals, produced by composite sets provided at the West terminal are applied to line l5 at the west terminal through composite set retardation coil III and line winding 51 of repeating coil I4 in series and are applied to line I6 through composite set retardation coil H2 and line winding II of repeating coil I4 in series.

Upon reaching the intermediate repeater section, the direct current signals present on line I5 pass through line winding H3 of repeating coil I! and retardation coil II4 to line H5 and those present on line I6 pass through line winding II6 of repeating coil I1 and retardation coil III to line I2I, lines H5 and I2I comprising a by-path over which the direct current signals are by-passed around the intermediate repeater section.

After by-passing the intermediate repeater section, the direct current signals are applied to line 42 through retardation coil I22 and line winding I23 of repeating coil M and are applied to line 43 through retardation coil I24 and line winding I25 of repeating coil 4|, the direct current signals now being transmitted over lines 42 and 43 to the east terminal.

Upon reaching the east terminal, the direct current signals pass respectively through line winding I26 of repeating coil 44 and composite set retardation coil I21 and through line winding I3I of repeating coil 44 and composite set retardation coil I32 to the direct current receiving equipment of the composite sets associated with the east terminal.

It will be understood that transmission of direct current signals over side circuit No. 1 from east to west andover side circuit No. 2 in both directions is accomplished ina manner similar to that described above.

II and retardation coil II2 are distorted. This It has been observed that, in the absence of the corrective means which is the subject of the present invention, signal distortion due to socalled cross-fire has resulted from passage of the direct current signals through the line windings of the repeating coils. To illustrate, let us refer to the west terminal of side circuit No. l and assume, for the moment, the absence of the corrective means of the present invention. It has been observed that, when direct current signals are passed through line winding 51 from the I composite set associated with line I5 through line winding 51 and retardation coil III, direct current signals passing from, or to, the composite set associated with line I6 through line winding distortion is due to transient currents which are set up by induction in line winding 1| due to the passage of the direct current signals through line winding 51.

The corrective means which is provided in accordance with the present invention is efiective in eliminating the distortion referred to. In accordance with this novel arrangement, inductance I33 is connected in shunt to the primary windings of repeating coil I4. This inductance offers a relatively low impedance to currents of low frequency and to direct current and, when connected as shown, results in a marked reduction of the coupling between windings 51' and II at low frequencies, or stated differently, the transformer efiiciency at low frequencies is lowered substantially. As the net result of this reduced coupling the currents set up in line Winding 'II by induction when direct current signals are passed through line winding 57 are of such a small value that their distorting effect on the signals passing from or to the composite set associated with line I5 is negligible.

It should be remembered that, while inductance I33 presents a low impedance to currents in the lower frequency range, the impedance offered to voice frequency currents is high from which it follows that provision of the corrective means has no appreciable effect on the efficiency of repeating coil I4 so far as the voice frequency currents are concerned.

It will be understood that inductance I33 also acts in the manner described to prevent the establishment of disturbing currents in line winding 51 when direct currents are passed through line winding II and it will be understood, further, that inductance I34 provided at the east terminal of side circuit No. 1, inductance I35 provided at the west terminal of side circuit No. 2 and inductance I36 provided at the east terminal of side circuit No. 2 all are effective in a manner similar to that described above in preventing signal distortion at the respective terminals with which they are associated.

Referring now .to Fig. 2, there is illustrated the west terminal of a speech transmission system of the general type illustrated in Fig. 1. The general arrangement of the terminal is the same as that of the terminals of Fig. 1 and the arrangementandfunction of parts common to both figures will not be again described. In the instance of the terminal of Fig. 2, however, capacitance 'I5I has been provided, being connected in series with terminating resistance I52. Provision of this capacitance at the terminal may be desirable, for example to modify voice frequency characteristics, or to block direct current signals froma certain path. In event of provision of the capacitance, the value ofinductanc'e I53 should be slightly less than that of inductance I33 of Fig. 1 as the capacitance reduces the shunting effect at low frequencies of the terminating resistance.

The capacitances provided in the phantom terminal and in the terminal of side circuit No. 2 are similar to and act in a manner similar to that provided in the terminal of side circuit No. 1.

While specific embodiments of the invention have been selected for illustration and detailed description, the invention is not, of course, limited in its application to such embodiments. The embodiments specifically disclosed should be looked upon as illustrative of the invention and not as restrictive thereof.

What is claimed is:

1. In a speech transmission system, a terminal section, a two-wire line, a repeating coil for inductively coupling said terminal section and said line, said repeating coil including a primary winding section and a secondary winding section, a composite set for transmission of direct current signals, the secondary winding section of said repeating coil including two inductive portions of high impedance to voice frequencies and a portion of low impedance to voice frequencies but of high impedance to direct currents, means for connecting said composite set across said latter portion, and a path connected in shunt to the primary winding section of said repeating coil, said path being of low impedance with respect to low frequency currents, including direct currents, but of high impedance with respect to voice frequency currents.

2. In a speech transmission system, a terminal section, a two-wire line, a repeating coil for inductively coupling said terminal section and said line, said repeating coil including a primary winding section and a secondary winding section, a composite set for transmission of direct current signals, the secondary winding section of said repeating coil including two inductive portions of high impedance to voice frequencies and a portion of low impedance to voice frequencies but of high impedance to direct currents, means for connecting said composite set across said latter portion, and an inductance connected in shunt to the primary winding section of said repeating coil, said inductance presenting low impedance to low frequency currents or direct current signals but high impedance to voice frequency currents.

3. In a speech transmission system, a terminal section, an intermediate repeater section, a twowire line for connecting said terminal section and said intermediate repeater section, means for inductively coupling said terminal section to the terminal end of said two-wire line, said coupling means including a first winding connected in shunt to said terminal section and conductive means connected in shunt to the terminal end of said two-wire line, said conductive means including a second winding, a capacitance and a third winding connected in series in the order named, said first winding and said conductive means being inductively coupled, said second and said third windings also being inductively coupled, a composite set for transmission of direct current signals, means for connecting said composite set across said capacitance, and means effective to materially reduce the coupling between said second winding and said third winding with respect to low frequency currents or direct currents, said last-mentioned means having substantially no effect on the coupling between said first and said second windings with respect to voice frequency currents.

4. In a speech transmission system, a terminal section, an intermediate repeater section, a twowire line for connecting said terminal section and said intermediate repeater section, means for inductively coupling said terminal section to the terminal end of said two-wire line, said coupling means including a first winding connected in shunt to said terminal section and conductive means connected in shunt to the terminal end of said two-wire line, said conductive means including a second winding, a capacitance and a third winding connected in series in the order named, said first winding and said conductive means being inductively coupled, said second and said third windings also being inductively coupled, a composite set for transmission of direct current signals, means for connecting said composite set across said capacitance, and means eflective to materially reduce the coupling between said second and said third windings with respect to low frequency currents or direct currents, said last-mentioned means comprising an inductance connected in shunt to said first winding and having substantially no efl'ect on the coupling between said second and said third windings with currents.

ELLIOT M. STAPLES. 

